Paper containing acrylamide copolymer



3,fi77,d3fi Patented Feb. 12, 1963 line 3,077,4536 PAPER CfiNTAlNlNGACRYLAMHDE CGPULYl/illlt dewell T. Moore, Stamford, Conn, assignor toAmerican Cyanamitl Company, New York, Niki, a corporation of Maine NoDrawing. Filed Sept. 25, 1959, Ser. No. 842,212 8 Claims. (Cl. 162-168)The present invention relates to paper of improved dry strengthsresulting from a content of a diallylmelamineacrylamide copolymer. Theinvention includes such paper containing rosin size, dyes, fillers,etc., together with the papermaking processes involved.

The term paper" as used herein includes cardboard, hardboard, tissue andall other waterlaid webs in substantial part composed of waterlaidcellulose fibers.

N,N-diallylmelamine (hereinafter termed diallylmelamine) is a relativelynew chemical compound which heretofore has found no utility in drystrength paper manufacture. It is insoluble in water, and does nothomopolymerize.

The discoveries have now been made that paper composed of waterlaidcellulose fibers carrying a small adsorbed amount of a normallywater-soluble dlallylmelamine-acrylamide copolymer possesses improveddry strength as compared with paper containing none of the copolymer. Ihave further found that such paper processes improved wet strength whenthe copolymer contains thermoset methylol groups, and that the presenceof the copolymer on rosin sized fibers improves the Water resistanceimparted by the size.

The invention is based on my discovery that watersolublediallylmelarnine copolymers are suificiently strongly cationic to beabsorbed in large amounts from aqueous solution by cellulose fiberssuspended therein, and that when the fibers carrying the copolymer aresheeted to form a web and the web is dried, the copolymer tends to bondlocally or spot weld the fibers together, thereby increasing the drystrength of the sheet.

In preferred instances the invention has increased dry tensile strengthvalues by more than 25%, and when a methylol diallylmelamine-acrylamidecopolymer has been employed, wet tensile strength values have beenobtained comparable to those possessed by commercial wet strengthpapers. Finally, the presence of the copolymer has nearly doubled theefiectiveness of rosin size when present, as measured by standardlaboratory sizing tests.

The paper of the present invention is prepared by f rming an aqueoussuspension of cellulose papermaking fibers, adding thereto a smallamount of a cationic watersoluble diallylmelamine-acrylamide copolymcr,sheeting the fibers to form a waterlaid web, and drying the web.

The process is applicable to any of the common furnishes. Groundwood,unbleached and bleached kraft and hardwood pulps have been found tobenefit from the treatment. The copolymer is advantageously added to thefibrous suspension as a dilute aqueous solution immediately before thesuspension is sheeted, for example at the fan pump. The copolymer israpidly and substantively adsorbed from solution by the fibers.

However, in the manufacture of rosin sized paper advantage is taken ofthe cationic nature of the polymer. Preferably, the copolymer is addedto the fibrous suspension first, and is substantively adsorbed. Thefibers are then rosin sized in an ordinary manner. Evidently theabsorbed polymer assists deposition of the rosin size, and beneficiallyaffects the amount and orientation of the copolymer deposited. Thecopolymer has no effect as a size, and the water resistance possessed byrosin sized paper containing diallylmelamine-acrylamide copolymer isgreatly superior to the water resistance of resin sized paper containingnone of the copolymer.

An important advantage of the use of the diallylmelamine-acrylamidecopolymer in conjunction with rosin size is that it permits formation ofpaper having normal dry tensile strength. Rosin sized paper isordinarily weaker than corresponding unsized paper.

A further advantage of the invention is that it permits the manufactureof rosin sized paper having an alkaline pH. Rosin sized paper ordinarilyhas an acid pH (about 4.5) and is therefore subject to acid tendering onaging. The paper of the present invention likewise may containacid-sensitive pigments such as ultramarine.

The copolymer may be added along or in conjunction with cationic wetstrength agents and cationic sizes. The process tolerates the additionof numerous materials which have been added to the beater, and thus itdesired, pigments such as clay, titanium dioxide, blanc fixe and calciumcarbonate may be incorporated prior to sheeting.

Paper according to the present invention is made by forming an aqueoussuspension of cellulose papermaking fibers, adding thereto a smallamount of a cationic Water soluble diallylmelamine-acrylamide copolymeras strengthening agent, sheeting the fibers to form a waterlaid web, anddrying the web. The copolymer is rapidly and substantively adsorbed bythe fibers.

A flow sheet of the invention in one preferred embodiment is as follows:

MM Form aqueous suspension of cellulose papermalcing fibers Add alum ascomplexing agent Sheet suspension to form a web Dry the web Paper ofimproved dry strength is obtained both fr m acid and alkaline pulps.Paper of somewhat better dry strength results when alum is added ascomplexing agent. The optimum amount is most easily found by laboratorytrial; generally enough alum to increase the pH f the suspension to4.5-5.0 after addition of the copolymer is enough.

Paper of improved wet and dry strength is obtained by employing acationic water-soluble thermosetting methylol diallylmelamine-acrylamidecopolyrner in the process described above, and sheeting the fibers at anacid pH to facilitate subsequent thermosetting of the copolymer.

Rosin sized paper is manufactured according to the present invention byadding a small amount of a cationic water-solublediallylmelaminc-acrylamide copolymer as strengthening agent to acellulose fiber suspension, rosinsizing the fibers by adding rosin sizeand alum, sheeting so /vane Jr the fibers to form a web, and drying theweb. In preferred instances the copolymer acts to counteract the nor malweakening etlect imparted by the size and to increase thewater-resistance and ink-resistance which the sheet would otherwisepossess. Moreover, the suspension may be sheeted at an alkaline pH toproduce alkaline paper.

The process tolerates the addition of such materials as have heretoforebeen added to paperrnaking pulps, and thus it is within the scope of thepresent invention to add clay filler, titanium dioxide, calciumcarbonate and ultramarine pigments, and dyes to the pulp beforesheeting.

The copolymer of the present invention is substantially composed ofvinyl units (hereinafter termed the diallylmelamine and acrylamidecomponents), having the theoretical formulae:

I NH2 and or methylol derivatives thereof, the acrylarnide componentbeing present in preponderant amounts, i.e., more than 50% by weight. Ifdesired, the copolymer may be exclusively composed of the unitsillustrated above, in which event the paper containing the polymer willhave substantially improved dry strength but low wet strength, so thatthe paper can be easily repulped without need for special mechanical orchemical treatment.

The copolyrner may contain methylol substituents formed by reacting someor all of the NH groups shown above with formaldehyde. Advantageously,the number i such groups present should be sufiicient to render thepolymer thermosetting. The minimum effective proportion for this purposemay be determined by heating the polymer briefly at 190 F. at a pH below5 Conversion of the copolymer to water-insoluble form shows thatsufficient methylol groups are present to render the copolymerthermosetting.

The copolymer may contain minor amounts of units other than thosedescribed above. Thus the copolymer may contain a minor amount ofcombined styrene, in which event the polymer will impart sizing inaddition to dry strength. Moreover, the polymer may contain hydrophiliccationic groups for example those introduced by chlorornethylstyrenequaternized with a water-soluble tertiary amine. The amount of suchsupplementary units should be insufficient to cancel the cationic,water-soluble character of the copolyrncr.

it is of particular interest that the copolymer may and oftenadvantageously will contain carboxylic groups such as are introduced bya combined content of acrylic acid, or by hydrolysis of some of theamide units present, the proportion of such carboxylic acid groups(which are anionic), however, being insufficient to change theessentially cationic character of the polymer as a whole. Polymers ofthis type (hereinafter termed diallymelamiue acrylarnide-acrylic acidcopolymers) form a complex with alum, and paper composed of cellulosefibers bonded together by this complex possesses better dry strengththan would otherwise be the case.

Paper composed of fibers carrying normally watersoluble cationiccopolymers prepared from dimethallylmelamine and acrylamide are withinthe scope of the invention.

The invention will be more particularly illustrated by the examples.These examples represent preferred embodiments of the invention and arenot to be construed as limitations thereon.

Example 1 The following illustrates the dry strength possessed by i aperaccording to the present invention composed of cellulose fibers bondedtogether by varying amounts of diallylrnelamine-acrylamide copolymerscontaining no methylol groups.

An unbleached southern kraft pulp was diluted to a consistency of 0.6%.A diallylmelamine-acrylamide copolymer (as a 5% aqueous solution) wasthen added in amounts shown below, alkali added to adjust the pH of thesuspension to 7.0, and the-suspension was sheeted to yield sheets havinga dry basis weight of 10-0 1b. (25 x 40"/500 ream). The wet sheets weredried at 240 C. 'for 1 minute, and their dry and wet tensile strengthsdetermined by standard laboratory methods.

None of the sheets had a wet tensile strength in excess of 3.8 lb. perinch and could be repulped by ordinary beater processing.

The copolymers used were prepared as follows. Diallylmelamine andacrylamide monomers were mixed in the ratio shown in the table below,0.15% based on the weight of the mixture of thiornalic acid addedthereto, and one part by weight of the mixture dissolved in 9 parts byweight of water at C. acidified to pH 2 with hydrochloric acid. Themixture was reacted at 7080 C. with slow addition of an aqueous solutioncontaining 1% based on the weight of the monomers of (NH S 0 asinitiator. The reactions were stopped by cooling and neutralization whenthe solutions were evidently near gelation.

Results were as follows:

Copolymor Dry Tensile Run No.

DAM :AM Percent Pulp Found, Percent Ratio 1 Added 9 pH 3 1b./in. Incr.

1 Initial (before polymerization).

2 Based on dry weight of fibers.

2 After addition of copolymer.

4 Corrected to 100 lb. basis weight.

The dry strength values obtained can be increased by 2-5 lb. by addingsufllcient alum after addition of the copolymer to decrease the pH ofthe suspension to 4.5, or by adding an equal amount at the outset.

Example 2 The following illustrates the dry and wet tensile strengthpossessed by paper containing a thermoset methyloldiallylmelamine-acrylarnide copolymer.

A diallylmelamine-acrylamide copolyrner was prepared by heating 50 gm.each of diallylmelamine and acrylarnide and 0.5 gm. of benzoyl peroxidein 400 gm. of dioxane in a flask equipped with agitator, refluxcondenser and electric heating mantle. At 88 C. cooling was applied tomaintain the reaction at 90 C. When the exotherm subsided, the flask wasmaintained at C. for 90 minutes. The polymer was filtered oft and washedwith dioxane. By analysis the ratio of components in the copolymer wasfound to be 28.91711, showing that the diallylrnelamine reacted moreslowly than the acrylamide.

The copolymer was methylolated by stirring 5.0 gm. of the copolymer intoa mixture of 16 gm. of 37% aqueous methanol-free formaldehyde and 224ml. of water,

heating rapidly to 80 (3., adding 5 ml. of 28% acetic acid, cooling to20 C. and diluting with water to 1% solids.

A series of paper sheets was prepared by the method of Example 1 exceptthat bleached northern kraft pulp was used, 2% of alum based on the dryweight of the fibers was added after addition of the copolymer givingthe pump a pH of 4.7, and the pulp was sheeted at a calculated basisweight of 48.5 lb. (25" x 40/500 ream).

A second series of sheets was prepared in similar manner except that thepulp was adjusted to the various pH values shown in the table beforesheenng.

Results were as follows:

Copolymer Dry Tensile Per- Wet Run. No. cent Pulp Ten- DAM AM PercentAlum pI-I Found, Persil Ratio 1 Added 2 Added lb./i.n.= cent 1b./in

Incr.

1 By analysis of eopolymer. 1 Based on dry weight of fibers. Correctedto 48.5 lb. basis weight.

Example 3 The following illustrates the effectiveness of small amountsof diallylmelamine-acrylamide copolymer in increasing the water and inkresistance imparted by rosin size.

A series of handsheets was prepared by the general method of Example 1employing bleached northern kraft pulp. To one aliquot was added 1% ofalum, to another aliquot was added wood rosin size and alum, and to thethird was added diallylmelamine-acrylarnide copolymer solution, woodrosin size and alum, in amounts shown in the table below.

The aliquots were then formed into handsheets which were tested by themethod of Example 1. Results were as follows:

1 Percentages based on dry weight of fibers.

The above data shows that paper produced by the use of 0.5% copolymerand 0.6% rosin size in comibnation possesses much better resistance topenetration by water and ink than paper produced by rosin size and alumalone, while being about 19% stronger.

Example 4 The following illustrates the dry strength and sizingproperties possessed by rosin sized paper of the present invention madeat acid and alkali pH values.

The procedure of Example 3 was repeated using a bleached sulfite pulpand copolymers formed by copo1ymerization of diallyl-melamine andacrylamide in 5:95, :90 and 20:80 weight ratios.

Results were as follows. The first three runs were controls.

Pulp Treatment 1 Sizing Results Run No. Poiy- Rosin Dry 11-30 In}:

mer, Size, Alum, pH Tensile, Resist. Resist. percent percent percentlb./in. Currier, BKY,

Sec. Sec.

Diallylmelamine :acrylarnide ratio 5:

Diallylmelamine:acrylamide ratio 10:90

Diallylmelamine :aerylamide ratio 20 80 X Percentages based on dryweight of the fibers.

I claim:

1. Paper of improved wet and dry strength composed of cellulose fiberslocally bonded together by a small adsorbed amount of a thermosetcationic, normally watersoluble thermosetting methyloldiallylmelamineacrylamide copolymer.

2. Paper of improved dry strength composed of celluose fibers locallybonded together by a small adsorbed amount of a complex of alum with acationic, normally water-soluble diallylmelamine-acrylamide-acrylic acidcopolymer.

3. Sized paper of improved dry strength composed of resin sizedcellulose fibers locally bonded together by a small adsorbed amount of acationic, normally water soluble diallylmelamine-acrylamide copolymer.

4. Paper according to claim 3 having an alkaline pH.

5. Process for manufacturing paper of improved dry strength, whichcomprises forming an aqueous suspension of cellulose papermaking fibers,adding a small amount of a cationic, water-solublediallylmelamine-acrylamide acrylic acid copolymer as strengthening agentthereby adsorbing said copolymer on said fibers, adding alum ascomplexing agent for said copolymer, sheeting said fibers to form awaterlaid web, and drying said web.

6. Process of manufacturing paper of improved wet and dry strength,which comprises forming an aqueous suspension of cellulose papermakingfibers, adding thereto a small amount of a cationic, water-solublethermosetting methylol diallylmelamine-acrylamide copolymer asstrengthening agent thereby adsorbing said copolymer on said fiber,sheeting said fibers at a pH to form a waterlaid web, and drying saidweb at a temperature between C. and 250 C. until said copolymer hasthermoset.

7. Process of manufacturing rosin size-d paper of improved dry strength,which comprises forming an aqueous suspension of cellulose papermakingfibers, adding thereto a small amount of a cationic, water-solublediallylmelamine-acrylamide copolymer as strengthening agent therebyadsorbing said copolymer on said fibers, rosinsizing the fibers carryingsaid copolymer, sheeting said fibers to form a waterlaid web, and dryingsaid web.

8. A process according to claim 7 wherein the fibers are sheeted at analkaline pH.

(References on following page) References 651%: in the file of thispatent UNITED STATES PATENTS House et a1. Ian. 3, 1956 5 8 Lundberg eta1. July 30, 1957 Wrotnowski Mar. 29, 1960 Stilbert et a1 Feb. 21, 1961FOREIGN PATENTS Canada Sept. 25, '1951 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3,077,430 February 12, 1963 SewellT. Moore corrected below.

Column 4 line 28, for "80% C, read 80 C., column 5, line 63, for"comibnation" read combination Signed and sealed this 11th day ofFebruary 1964c (SEAL) Attest:

' EDWIN Lo. REYNOLDS ERNEST W SWIDER Attesting Officer Ac ti ngCommissioner of Patents

5. PROCESS FOR MANUFACTURING PAPER OF IMPROVED DRY STRENGTH, WHICHCOMPRISES FORMING AN AQUEOUS SUSPENSION OF CELLULOSIC PAPERMAKINGFIBERS, ADDING A SMALL AMOUNT OF A CATIONIC, WATER-SOLUBLEDIALLYLMELAMINE-ACRYLAMIDE ACRYLIC ACID COPOLYMER AS STRENGTHENING AGENTTHEREBY ADSORBING SAID COPOLYMER ON SAID FIBERS, ADDING ALUM ASCOMPLEXING AGENT FOR SAID COPOLYMER, SHEET SAID FIBERS TO FORM AWATERLAID WEB, AND DRYING SAID WEB.